We have continued our studies of the insulin-like growth factors (IGFs), their receptors and carrier proteins. During the past year we have demonstrated that: (1) multiple IGF-II RNAs (1.2 to 4.6 kb) arise from a single gene through the use of 2 promoters and 2 polyA addition sites; (2) the IGF-II gene is transcribed from both promoters in 11 fetal rat tissues; (3) IGF-II RNA in different tissues is translated into pre-pro-rIGF-II is processed to 7.5 kDa IGF-II; (4) the fetal/neonatal form of the IGF carrier protein has been cloned; (5) the carrier protein precursor contains a 34 amino acid prepeptide and the 270 amino acid mature protein; (6) carrier protein mRNA is expressed in multiple fetal tissues, but is decreased in the corresponding adult tissues; (7) polyclonal antibodies to the purified type II IGF receptor do not stimulate or inhibit IGF actions in L6 rat myoblasts, suggesting that these effects are mediated by the type I rather than the type II receptor, (8) circulating type II IGF receptors corresponding to the extracellular domain are present in fetal and neonatal rat serum; (9) levels of the circulating type II receptor decrease markedly in older rats; (10) activation of human T lymphocytes results in increased expression of type I and type II IGF receptors, suggesting that the IGFs may participate in the activation cascade; (11) two-chain insulin-IGF hybrid molecules containing the A-domain of IGF-I have increased mitogenic activity and binding to type I IGF receptors but do not bind to IGF carrier proteins.